1.5V LED Flasher Oscillator

Mimics the now discontinued LM3909 IC

Can be adapted to a wide variety of applications

Design description:

The 8-lead plastic mini-DIP LM3909 IC was developed by National Semiconductor in the mid 'seventies of the past century. It was a monolithic oscillator specifically designed to flash Light Emitting Diodes. By using the timing capacitor for voltage boost, it delivered pulses of 2 or more volts to the LED while operating on a supply of 1.5V or less. The circuit was inherently self-starting, and required addition of only a battery and capacitor to function as an LED flasher.
Unfortunately, since 1998, the manufacturer discontinued the production of this chip. For this reason, and on request of some correspondents, I tried to emulate this IC operation using common discrete components, obtaining unexpected satisfactory results.
Obviously, this circuit requires more room on the PCB than the original IC, but it is perhaps a bit cheaper, yet performing in a very similar manner.

The following table shows the features of several LED flasher circuits obtained by changing C or R parts (drawn in red color in the diagram) and voltage supply. The LED (D) should be preferably of the red type.

Circuit type

C value

R value

Battery voltage

Nominal flash rate

Average current drain

1.5V Flasher

330µF 6V

3K3 1/4W

1.5V

1 Hz

0.64mA

Low power 1.5V

100µF 6V

8K2 1/4W

1.5V

1.2 Hz

0.32mA

Fast Blinker

330µF 6V

1K 1/4W

1.5V

2.6 Hz

1.2mA

3V Flasher

330µF 6V

8K2 1/4W

3V

1 Hz

0.77mA

6V Flasher

330µF 6V

10K 1/4W

6V

1.5 Hz

0.7mA

Note: The flashing rate can be different from nominal values shown above: it should be remembered that some electrolytics have very broad capacitance tolerances, for example -20% to +100%.

Comments:

The above diagram shows a higher power application such as would use an automotive storage battery for power. It provides about a 1 Hz flash rate and powers a lamp drawing a nominal 600 mA.
A particular advantage of this circuit is that it has only 2 external wires and thus may be hooked up in either of the two ways shown in the diagram below. Further, no circuit failure can cause a battery drain greater than that of the bulb itself, continuously lit.

Notes:

The circuit should operate safely in the 6V-14V range, provided the bulb used has the same nominal voltage of the supply used.